Role of Slip Step Emergence in the Early Stages of Stress Corrosion Cracking in Face Centered Iron-Nickel-Chromium Alloys

Abstract

Thin foils of four face centered cubic iron-nickel-chromium alloys and of pure iron and pure nickel have been examined in the electron microscope before and after exposure to boiling MgCl₃. Evidence of parallel dissolution arrays in stressed thin foils was taken as confirmation of formation of slip steps by dislocations which move while specimens are in intimate contact with an active environment. The faulted region exposes non-protected base metal with the result that rapid local dissolution occurs along an axis of the active slip plane. Because parallel dissolution arrays occur in susceptible as well as non-susceptible alloys, the process of slip step activated dissolution is taken to be necessary, but not critical in stress corrosion cracking. Evidence is presented to suggest that neither dislocation coplanarity (as shown in thin foils) nor dislocation reactivity are factors in stress corrosion cracking in austenitic Fe-Ni-Cr alloys. The emergent slip step model is hypothesized to be useful in interpreting propaga­tion phenomena and grain size effects in stress corrosion cracking susceptibility.